IonMonkey, the new JavaScript engine Mozilla has been building for Firefox, is now just a single release away from making its way to the stable release channel. Firefox 18 arrived for beta users this week, and it’s bolted on IonMonkey as well as several other fairly major additions.

Firefox 18 will finally catch up to Safari and Opera on OS X by adding support for Retina displays, and Mozilla’s open source PDF.js reader has brought integrated viewing of Adobe‘s ubiquitous document format. Mozilla has also added support for W3C touch events, which should help make Firefox 18 a better fit on the numerous Windows 8 convertibles, touchscreen notebooks, and all-in-ones that are now arriving on store shelves.

There’s also been one key security-related update: Firefox 18 can now disable insecure content on sites that are being accessed via HTTPS. It’s a privacy protection feature, and will prevent those insecure connections from leaking data. To turn it on, head to about:config and search for allowinsecure. Make sure network.websocket.allowInsecureFromHTTPS is set to false — it appears to be by default, but it never hurts to check.

Mozilla has done a good job of keeping pace with Google in terms of functionality in recent times, and with IonMonkey now on board, Firefox has taken a big step forward in performance, too. In a quick run through the Sunspider benchmark, Firefox 18 put up a time that was just 15ms slower than the Chrome 24 beta. And while solid benchmark results don’t always translate to gains on the real Web, Firefox 18 definitely seems to handle JavaScript-heavy websites better than the previous version did. Gmail fires up faster, for example. Perhaps not blisteringly so, but there’s enough of a difference that you’ll notice.